Rapidly flashing or rotating blue, red, white, amber or green (depedning on the state) halogen, strobe or L.E.D lights found on the vehicles of volunteer fire fighters and EMTs to be used in responding to the scene or the squad building\firehouse. Most states refer to them as "courtesy lights" which means the volunteer is asking traffic to pull over to let him by but they are not required to by law. In some states, volunteers responding in their personal vehicles are supposed to be given the same right of way as an Emergency Vehicle. Some volunteers don't use them (such as those that live close to the building) and some have just one, placed in the center of the dashboard or windshield. A volunteer who is considered a whacker may have half a dozen or more of these flashing and rotating lights. Hence the courtesy light became known as Whacker Lights.

(Brightly lit vehicle with flashing blue lights flys by as two men stand outside on the sidewalk)
Neighbor1: What the hell was that?!
Neighbor2: That was John from 106, he's with the fire department. He's got a dozen and half of those Whacker Lights on his pickup.
Neighbor1: Fuck, that's bright!